Functionalized S-thio-di- and S-oligosaccharide precursors as templates for novel SLex/a mimetic antimetastatic agents

Biological properties of (1-4)-thio disaccharides

Thio sugars are carbohydrate derivatives in which one or more oxygen atoms have been replaced with sulfur. Thio sugars are effective inhibitors of glycosylases, have considerable therapeutic potential, and are used as drugs in the treatment of diabetes and infectious diseases. The development of this branch of carbohydrate chemistry would not be possible without the development of novel methods for its synthesis and the analysis of their biochemical properties. In this Review Article, we summarize our findings on the biological properties of a collection of thio sugars and their derivatives synthesized by the Witczak and Bielski team using their original methods based on the Michael addition of sugar thiols to levoglucosenone.

Cellulose-Derived Functional Polyacetal by Cationic Ring-Opening Polymerization of Levoglucosenyl Methyl Ether

The unsaturated bicyclic acetal levoglucosenyl methyl ether was readily obtained from sustainable feedstock (cellulose) and polymerized by cationic ring-opening polymerization to produce a semicrystalline thermoplastic unsaturated polyacetal with relatively high apparent molar mass (up to ca. 36 kg mol-1 ) and decent dispersity (ca. 1.4). The double bonds along the chain can undergo hydrogenation and thiol-ene reactions as well as crosslinking, thus making this polyacetal potentially interesting as a reactive functional material.

Straightforward synthesis of thiodisaccharides by ring-opening of sugar epoxides

3,4-Anhydro hexopyranosides have been prepared by diastereoselective epoxidation of derivatives of 2-propyl 3,4-dideoxy-alpha-D-erythro-hex-3-enopyranoside (5), selectively protected at HO-2 and HO-6. The allylic group at C-2, in 5 and derivatives, plays a critical role in the facial selectivity of the epoxidation reaction. Thus, the free HO-2 in 3 (the 6-O-acetyl derivative of 5) directs the attack of m-chloroperbenzoic acid from the more hindered alpha face of the molecule to give 2-propyl 6-O-acetyl-3,4-anhydro-alpha-D-allopyranoside (7) accompanied by the beta epoxide 6 as a very minor product. Reverse diastereoselectivity has been obtained when the HO-2 in 3 was substituted by a bulky tert-butyldimethylsilyl (TBS) group. In this case, the major isomer was the 2-O-TBS derivative of 6 (alpha-D-galacto configuration). The ring-opening of sugar epoxides by nucleophilic per-O-acetyl-1-thio-beta-D-glucopyranose (11) was employed as a convenient approach to the synthesis of (1-->3)- and (1-->4)-thiodisaccharides. For example, ring-opening of the oxirane 7 by 11 led to the expected regioisomeric per-O-acetyl thiodisaccharides beta-D-Glc-S-(1-->3)-4-thio-alpha-D-Glc-O-iPr (12) and beta-D-Glc-S-(1-->4)-4-thio-alpha-D-Gul-O-iPr (13). Regioselectivity in the construction of the (1-->4)-thioglycosidic linkage could be achieved by hindering C-3 of the 3,4-anhydro sugar with a bulky silyloxy group at the vicinal C-2. For instance, coupling of the 2-O-TBS derivative of 7 with 11 led regioselectively to the protected thiodisaccharide beta-D-Glc-S-(1-->4)-4-thio-alpha-D-Glc-O-iPr (27). The utility of the approach was demonstrated through the synthesis of sulfur-linked analogues of naturally occurring (laminarabiose and cellobiose) and non-natural disaccharides (i.e., beta-D-Glc-(1-->4)-alpha-D-Gul).

A click chemistry approach to glycomimetics: Michael addition of 2,3,4,6-tetra-O-acetyl-1-thio-β-d-glucopyranose to 4-deoxy-1,2-O-isopropylidene-l-glycero-pent-4-enopyranos-3-ulose – a convenient route to novel 4-deoxy-(1→5)-5-C-thiodisaccharides

The base catalyzed conjugate Michael addition of the 1-thiosugar, 2,3,4,6-tetra-O-acetyl-beta-D-glucopyranose, 1, to a new highly reactive enone 4-deoxy-1,2-O-isopropylidene-L-glycero-pent-4-enopyranos-3-ulose, 2, proceeds steroselectively with formation of adduct 3 in 94% yield. Convenient stereoselective reduction of the C-3 keto function of 3 with L-Selectride followed by in situ acetylation produces thiodisaccharide 4 in good 82% yield. Cleavage of the 1,2-O-isopropylidene protecting group with p-toluenesulfonic acid in methanol, followed by de-O-acetylation, produced an inseparable anomeric mixture of methyl 4-deoxy-5-C-(beta-D-glucopyranosyl)-thio-alpha/beta-L-ribo-pyranoside 5 in 72% overall yield. This approach constitutes a new general two-step click chemistry route to the previously unknown class of 4-deoxy-(1-->5)-5-C-thiodisaccharides as stable and biologically important glycomimetics.

Regioselective synthesis of 6-S-alkyl and 6-S-glycosyl-6-thio-D-mannofuranose derivatives from 5,6-O-cyclic sulfate precursors

C-6 opening of 5,6-cyclic sulfate derivatives of mannofuranose with a thiolate anion followed by acidic hydrolysis of the acyclic sulfate gave 6-S-alkyl derivatives in good yields (70-95%) and short reaction times (10-15min). This methodology was applied to the synthesis of methyl 2,3-O-isopropylidene-6-S-(2,3,4,6-tetra-O-acetyl-beta-d-glucopyranosyl)-6-thio-alpha-d-mannofuranoside (70%), 2,3-O-isopropylidene-6-S-(2,3,4,6-tetra-O-acetyl-beta-d-glucopyranosyl)-6-thio-alpha-d-mannofuranose (87%) and 2,3-O-isopropylidene-6-S-(1,2:3,4-di-O-isopropylidene-alpha-d-galactopyranos-6-yl)-6-thio-alpha-d-mannofuranose (87%).

Thiosugars: new perspectives regarding availability and potential biochemical and medicinal applications

Thiosugars, containing a sulfur atom as heteroatom or a disaccharide linked via a sulfur bridge, possess unique physicochemical properties such as water solubility, which differs from conventional functionalized monosaccharides. The differences in biological activities between thiosugars and their oxygen analogs depend on geometric, conformational, and flexibility differences. They depend also on their electronic differences, the sulfide function being less electronegative and more polarizable than the ethereal moiety. Many functionalized thiosugars occur naturally and are potential targets for the development of carbohydrate-based therapeutics. Among the few new examples of the potential new targets are salacinol and kotalanol, tagetitoxin, thiolactomycin and analogues, mycothiol and analogues, and S-nitrosothiols. These new developments and representative examples of functionalized thiosugar prototypes as potential new targets are presented in this mini review.